Reproduction motor cap for Featherweight 221.  This is the motor cap screw that measures 3/8" across the bottom*.  It covers the carbon brush inside the motor.  There are actually two motor caps on your Featherweight motor, because there are two carbon brushes - one is seen clearly on the top of the motor, but the other one is hidden on the underside. You can access the bottom motor cap by inserting a screwdriver into the allotted hole on the bottom of the machine or by removing the motor altogether. 

You can see in the photo above how the broken one needs to be replaced. You can click here  to learn how to replace a broken motor cap screw. 

* If your motor cap is larger and measures 1/2" across the bottom (more common to White 221K and 222K Free-Arm Featherweight motors), there is not a replica part available.